Vehicle heaters which generate heat by combustion and thus represent a heat source for the vehicle have been known for quite a long time. The heater supplies heat for an interior space of a vehicle and/or for preheating the internal combustion engine of the motor vehicle prior to cold start. Heat generation for an interior space of a vehicle may be heat generation with the internal combustion engine not running ("space heating") or auxiliary heat generation in the case of insufficient heat supply from the internal combustion engine ("supplementary heating").
In many cases, the heater is connected into the coolant circuit of the vehicle engine, especially because both preheating of the engine and heating of the interior space are thus possible, and because the vehicles own warm air distribution system can be optimally utilized for the interior space of the vehicle. The most efficient subdivision of the heating capacity of the heater between the interior space of the vehicle and the internal combustion engine was the object of many different considerations and solutions, especially also because, depending on the conditions of use and the weather conditions, various aspects were in the foreground. Providing a bypass line leading past the internal combustion engine and using a valve to control whether the coolant flow passes through the internal combustion engine, the bypass line, or, in defined quantity ratios, through both the internal combustion engine and the bypass line, has proved to be a favorable solution. When the coolant flow passes completely through the bypass line, the heating capacity of the heater is concentrated on use for heating the interior space of the vehicle; the internal combustion engine itself is not heated at all. When the coolant flow passes completely through the internal combustion engine, it receives maximum heating; the heating capacity used to heat the internal combustion engine reduces the heating capacity available for heating the interior space of the vehicle.
The prior-art valves used in the coolant circuit described are thermostat valves which regulate the return temperature in the coolant circuit within a predetermined range of adjustment. The return temperature is the temperature with which the coolant passed through the internal combustion engine and/or the bypass line is returned to the heater in order to be again heated there. The prior-art thermostat valves operate so that coolant is fed to the internal combustion engine only when its temperature is higher than the temperature set point set by the range of adjustment of the thermostat valve in the return of the coolant circuit. Consequently, the thermostat valve always sends to the internal combustion engine only just enough heat for heating it as is superfluous in the return with respect to the temperature set point. Such an engine preheating is especially suitable under cold start conditions. If the outside temperatures are high enough, i.e., when preheating the engine is unnecessary or when the engine still has sufficient residual heat from preceding operation for starting at lower outside temperatures, it is unfavorable from the viewpoint of energy utilization to heat the internal combustion engine much more intensely than would be necessary, because of the established temperature set point of the thermostat valve the heater in the coolant circuit would have to be operated at a higher output to heat the interior space of the vehicle.